Among sensors, there are capacitance-coupled sensors, namely capacitance-coupled (electrostatic capacity) sensors using a condenser (capacitor). JP-A-2000-80703 discloses a sensor using a capacitance-coupled sensor for detecting the human body sitting on a toilet seat of a human body private part washing device. The sensor disclosed in the official gazette has a sensor unit comprising a detection electrode and an earth electrode for capacitive coupling, and a protection electrode interposed between the detection electrode and the earth electrode, and the electrodes are insulated from each other. The electrostatic capacity between the detection electrode and the earth electrode is used to set the oscillation frequency of a high-frequency oscillator, and a change in the electrostatic capacity caused by approach of a human body is obtained as a change in the oscillation frequency (final output signal) from the oscillation circuit. The protection electrode is provided to reduce the electrostatic capacity possessed by the sensor itself.
However, the sensor disclosed in the official gazette requires a special device for detecting frequency since the output signal obtained from it a signal representing a change in frequency, which is not common. Also, the sensor disclosed in the official gazette is to detect the presence or absence of a substance that is a human body and is not intended to detect changes in the amount of a substance such as a liquid in a continuously variable manner.
This invention has been made in view of the above circumstances and it is, therefore, an object of this invention to provide a capacitance-coupled sensor from which a general DC voltage signal is obtained and a method for detecting a substance using the capacitance-coupled sensor.